It's far easier to go through the gender reassignment process if secondary sexual characteristics haven't developed, it can reduce the numbers of operations a person would need, for example female to male wouldn't need bilateral mastectomy, and the general female or male 'shape" would not be established making hormone therapies more effective as the process would start from a neutral midpoint.
The problem comes when puberty comes earlier, before the concept of Gillick competency comes into play. Parents may block the choice of their son of daughter , which may in rare cases not be in their son of daughter's best interest. It's a difficult decision, but if it means a young person has more time to decide that they really are in the wrong body as it were then it may have a role in the care of these children who already have a horrendously difficult time.